1. Field
The invention presented here concerns a rotation lock device located in the area of two separate, independent shafts or shaft sections which is designed to prevent undesired rotation of a shaft or part of a shaft. Rotation lock devices of this type can be used in various appliances in which a tool is driven via shafts. As an example of one possible area of use, which is in no way to be interpreted as a restriction on the total possible fields of use, the rotation lock device is presented below in a contra-angle for prophylactic dental treatment.
2. Description of Prior Art
The aim of preventative dentistry, so-called prophylactic dental treatment, is the prevention of diseases associated with the teeth, mouth and gums. Apart from prophylactic treatment carried out by the individual (cleaning one's teeth at home, for example) the dentist can also perform prophylactic treatment, e.g. removing tartar, plaque and discoloration, or teeth polishing. The latter is mainly carried out using special instruments, e.g. contra-angle and specific tools, such as caps, so-called ‘prophy-cups’ or brushes. These are secured in the head of the contra-angle by means of a tool coupling and driven by one or more shafts which are connected to a motor.
The tools used are equipped with a threaded pin end which is used to attach the tool to the head of the contra-angle. This pin is screwed into the upper drive (the coupling shaft), which is equipped with the corresponding thread. The upper drive is connected to the motor via at least one other shaft, so that the prophy-cup rotates when the motor runs.
In order to guarantee safe operation the prophy cup must be securely connected to the contra-angle on the one hand, and it should be possible to replace the cup quickly and easily on the other hand. This is necessary for hygienic reasons, as the cup should be replaced (at least) between treatments of different patients. However, sometimes replacement is also carried out during treatment of the same patient.
Tool removal, however, turns out to be extremely difficult in the case of those handpieces or contra-angles presently available on the market: The tool is fitted into the coupling by screwing it in the opposite direction to motor-driven shaft rotation. This is necessary in order to prevent the tool from becoming loose and unscrewing itself out of the coupling during operation, as the pressure on the treatment surface during application and the resulting friction forces act as a brake on the prophy-cup and this then tends to rotate more slowly than the coupling shaft.
On the other hand, however, screwing the tool into the coupling in the opposite direction to shaft rotation means that the tool is often screwed still more tightly into the coupling during treatment, and in order to remove the tool it has to be turned in the same direction as the shaft rotates during operation. The force required to unscrew the tool, which is extremely firmly seated in the head at the end of treatment, is thus often greater than the force required to move the drive shafts. For this reason, when attempting to unscrew the tool from the coupling by hand, the user very often only succeeds in rotating the shafts without loosening the tool.
Treatment paste, which sticks to the tool after the treatment and makes it slippery, makes it even more difficult for the user to remove the tool.
In disclosure document WO 99/12491 A1, a device is described which prevents the drive shaft rotating when the tool is being removed. A pin, which protrudes radially from the front shaft section, is fed into a sliding bearing assembly which is fitted with a helical bore and mounted around the rear drive shaft section. When an attempt is made to remove the tool and this causes the front shaft section to rotate, the pin is turned until it is positioned at the highest point of the helical bore. This causes axial displacement in both shaft sections or shaft extension, which presses against the two bearings. The resistance thus created is sufficient to prevent rotation of the shaft and this makes it possible to unscrew the tool from the coupling.
One disadvantage of this device is that the axial displacement of the shaft sections subjects the bearings to a high loading. In the case of ball bearings, the rollers in the bearing race are compressed, which causes a deterioration in running characteristics. Repeated loading may finally lead to the collapse of the bearings and to related servicing and replacement work, which is expensive both in terms of time and money.
For this reason the invention presented here is based on the task of creating an improved device for preventing the undesired rotation of the shafts when, for example, a tool is being unscrewed from the coupling.